A current shunt provides for indirect measurement of current values by measurement of the voltage developed across the current shunt by the current passing through the current shunt. Typical applications for current shunts include electricity usage control, over-current protection and metering of electricity consumption and generation. In use a current shunt of known resistance is provided in series with a load and the voltage developed across the current shunt by the load drawn current is measured. The current passing through the current shunt is then determined on the basis of Ohm's Law in view of the measured voltage and the known resistance of the shunt.
Certain applications, such as metering of electricity consumption and generation, require measurement to high accuracy over extended periods of time. For example in North America the ANSI C12.20 standard specifies an accuracy of ±0.5% for Class 0.5 consumption meters and ±0.2% for Class 0.2 consumption meters. Standards applicable in Europe and elsewhere, such as IEC 62053, specify similar accuracy requirements. It can therefore be appreciated that the resistance of the current shunt must be known to high precision to enable a meter to meet regulated accuracy requirements. Although the shunt resistance is normally low to minimise power dissipation and undesirable circuit effects, the current shunt is nevertheless liable to heating with temperature drift giving rise to a change in resistance which may cause a loss of measurement accuracy in a shunt of ordinary temperature coefficient of resistance. Shunt resistors formed from manganin alloy are therefore widely used in view of their very low temperature coefficient of resistance. It may also be apparent that accurate current measurement depends on measurement of the voltage developed across the shunt being accurate and stable with temperature and lifetime. This is because a change in the transfer gain of the voltage measurement circuit or lack of precision in references used in the voltage measurement circuit will cause an error. It is normal for these reasons to perform a one-off factory calibration when the current shunt and the readout electronics are combined so that a factor related to the actual combined transfer function for current to measurement value, which is determined largely by the shunt resistor and voltage measurement, can be stored and used in subsequent measurements to achieve the desired precision.
An alternative known approach to measuring high values of current involves the use of a current transformer wound on a core, which is disposed around a conductor carrying current to be measured. The current transformer has the advantages over the shunt resistor of being less invasive and providing for isolation from the current carrying conductor. The current transformer is capable of measuring AC current only. The current transformer generates a current in the secondary coil, which is a ratio of the current in the primary conductor, and the secondary coil current is then turned into a voltage by a load, known as a burden resistor. Accurate measurement of the voltage across the burden resistor and accurate knowledge of the transfer function of the primary current to voltage across the burden resistor (i.e. combining the effect of number of turns, the magnetics and the burden resistor) are needed to measure the current accurately and precisely. As with the current shunt, one-off factory calibration is often performed to compensate for inaccuracies in some or all of the elements that contribute to the overall transfer function of primary current to measurement value.
Another approach uses a Hall current probe which is capable of measuring both AC and DC. In an open loop configuration the Hall current probe is, however, liable to non-linearity and temperature drift. In a closed loop configuration the Hall current probe provides an improvement with regards to non-linearity and temperature drift although the weight and size of the configuration increases significantly where higher currents are measured. It is further known to use the Rogowski coil current probe to measure high levels of current. Most known approaches to current measurement, such as by way of the shunt resistor, the current transformer, the Rogowski coil and the Hall current probe, are described and discussed in Current Sensing Techniques: A Review, Silvio Ziegler Robert C. Woodward and Herbert Ho-Ching Iu, IEEE Sensors Journal, Vol. 9, No. 4, April 2009. The different known approaches have their respective advantages and disadvantages.
Load current measurement is often made in conjunction with line voltage measurement, which involves measuring the voltage between the conductors over which the current is delivered, in order to determine the electrical power. Often a resistive potential divider between the conductors is employed for line voltage measurement. High accuracy power calculation requires accurate and stable relative phase and frequency response of load current and line voltage measurements in order to accurately determine metrics such as the like of power factor, harmonic content and differences between active and reactive power amongst other things.
WO 2013/038176 describes an improved approach to the measurement of current. According to the approach of WO 2013/038176 a current sensor, such as a current shunt, a current transformer, a Hall current probe or a Rogowski coil, is disposed as described above relative to a conductor to sense a load drawn current flowing through the conductor. A reference signal which is known to high precision is applied to the current sensor whereby the current sensor is responsive to both the load drawn current signal and the applied reference signal. The output signal from the current sensor is acquired and the part of the output signal corresponding to the reference signal is extracted from the output signal. Then the transfer function of the current sensor and the current sensor processing chain is determined on the basis of the reference signal and the extracted part of the output signal corresponding to the reference signal. Thereafter the actual load drawn current flowing through the conductor is determined in dependence on the transfer function and the load drawn current as sensed by the current sensor. Accuracy of measurement of the load drawn current therefore depends on the reference signal being known to high precision instead of the current sensor and its processing chain being known to high precision as according to the previously described approaches. The lack of reliance on the known precision of the current sensor means a lower quality sensor may be used. There is also less need for initial calibration and periodic subsequent recalibration of the current sensor and its processing chain. Furthermore the approach of WO 2013/038176 addresses drift of the current sensor and its processing chain arising from the like of ageing and temperature change and also provides for additional functionality, such as the detection of tampering with electricity consumption meters.
The approach of WO 2013/038176 relies on proper extraction of the part of the output signal corresponding to the reference signal from the rest of the output signal. According to WO 2013/038176 a reference signal in the form of a sine wave is applied to the current sensor. Frequency domain analysis, such as FFT analysis, of the output signal is used to separate the part of the output signal corresponding to the reference signal from the rest of the output signal.
The present inventors have recognised that an increase in the power of the applied reference signal is beneficial, for example, in respect of the signal to noise ratio of the extracted part of the output signal corresponding to the reference signal. Furthermore a square wave carries more power than a sine wave for the same amount of power drawn from the power supply. A reference signal in the form of a square wave therefore carries more power than a reference signal in the form of a sine wave for the same power drawn from the power supply. The present inventors have further recognised, however, that extraction of the part of the output signal corresponding to a square wave reference signal from the output signal by frequency domain analysis is liable to be computationally intensive because one needs to take the plural tones of the square wave into account.
In view of the foregoing considerations the present inventors have concluded that time domain processing of the output signal may be preferable to frequency domain processing of the output signal. This conclusion, however, leads to the further problem of how to extract the part of the output signal corresponding to the reference signal from the rest of the output signal. This problem presents a particular challenge because in the current measurement apparatus of WO 2013/038176 the amplitude of the reference signal is normally much smaller than the amplitude of the load drawn current signal comprised in the rest of the output signal.
A potential attenuator provides for measurement of voltages in apparatus where the electrical signals are of larger magnitude than the acceptable dynamic range of the measurement apparatus. For example in mains electricity metering, where the AC line voltage between the live conductor and neutral conductor is 230V RMS and the measurement apparatus is restricted to a few volts DC relative to one of the conductors, a potential attenuator with a division ratio of a few hundred to a few thousand allows the measurement apparatus to determine the line voltage depending on knowledge of the attenuation factor of the potential attenuator and the gain of the measurement apparatus. Similarly in DC apparatus such as a battery monitor a potential attenuator allows for the battery voltage, which may be greater than 10 volts, to be accommodated by measurement apparatus having a supply voltage in the few volts range provided a transfer function of the measurement apparatus including the potential attenuator is known. This puts a requirement on accuracy and precision of both the potential attenuator and the measurement apparatus. In its simplest form the potential attenuator is a resistor divider comprising a large first resistor and a smaller second resistor which are connected in series between the two conductors. The accuracy of such a potential attenuator is determined by the attenuation factor of the first and second resistors being known, e.g. from measurement during calibration of the whole apparatus. The subsequent precision of the attenuation factor depends on the relative change of the resistors' values under changing conditions such as temperature and ageing. Maintaining accuracy over lifetime whilst being subject to environmental change imposes requirements on the resistors in terms of temperature coefficients, power handling, physical location, local stress and other factors that affect resistance values. Additionally the measurement apparatus is required to be stable with temperature and lifetime and other environmental changes. A more complex form of potential attenuator comprises an active circuit. For example this form of potential attenuator comprises an amplifier in an inverting configuration with a large input resistor and a small feedback resistor joined at the virtual earth of the amplifier. The virtual earth of the amplifier is electrically coupled to a first conductor and the first resistor is electrically coupled to a second conductor whereby the potential attenuator is operative to attenuate the voltage between the first and second conductors.
WO 2014/072733 describes an improved approach to the measurement of voltage. One configuration of measurement apparatus described in WO 2014/072733 measures voltage between a first conductor and a second conductor. The measurement apparatus comprises a potential attenuator, which has a first resistor and a reference resistor arrangement, and an offset voltage circuit. The offset voltage circuit comprises, for example, a passive resistor divider in series with an offset voltage. The measurement apparatus is configured such that the offset voltage circuit applies an offset voltage between the first conductor and the second conductor. The offset voltage is switched between values or is modulated with a more complex signal to create a reference input signal, which affects the potential attenuator by the same attenuation factor as for voltage measurement between the first and second conductors. The reference input signal which is known to high precision is applied to the potential attenuator, which is responsive to both the line voltage signal between the first and second conductors and the applied reference input signal. The output signal from the potential attenuator is measured and the reference output signal, which is the part of the output signal corresponding to the reference input signal, is extracted from the output signal. Then the overall transfer function of the potential attenuator and the voltage processing chain is determined on the basis of the reference input signal and the extracted part of the output signal corresponding to the reference input signal. Thereafter the actual voltage between the first and second conductors is determined in dependence on the transfer function and the line voltage measured by the potential attenuator. Accuracy of measurement of the line voltage therefore depends on the reference signal being known to high precision instead of the potential attenuator and its processing chain being known to high precision as according to previously described approaches. The lack of reliance on the known precision of the potential attenuator means lower quality components can be used. There is also less need for initial calibration and periodic subsequent recalibration of the potential attenuator and its processing chain. Furthermore the approach of WO 2014/072733 addresses drift of the potential attenuator and its processing chain arising from the like of ageing and temperature change and also provides for additional functionality, such as on-line monitoring of deployed accuracy of the voltage measurement or the detection of faults in electricity consumption meters.
The measurement apparatus of WO 2014/072733 in common with the measurement apparatus of WO 2013/038176 relies on an ability to extract the reference output signal from an output signal from the measurement apparatus. The output signal also contains a line voltage output signal corresponding to the electrical signal being measured (i.e. current or line voltage) which may have components similar in nature to the reference input signal and which may be more than an order of magnitude greater than the reference output signal. WO 2014/072733 would similarly benefit from reliable time domain extraction of the reference output signal.
The present invention has been devised in the light of the above described problems. It is therefore an object for the present invention to provide electrical measurement apparatus which is configured to provide for accurate measurement of an electrical signal, for example a mains current signal or a mains line voltage signal, in dependence on time domain processing. It is another object for the present invention to provide a method of measuring an electrical signal accurately, for example a mains current signal or a mains line voltage signal, in dependence on time domain processing.